1. Field of the Invention
The present invention relates to a device for inspecting the height of wires bonded between pads of semiconductor chips and leads on lead frames and more particularly to a bonding wire height inspection device for inspecting bonded wires that exceed the allowable maximum wire loop heights.
2. Prior Art
Japanese Patent Application Pre-Examination Publication (Kokai) No. 8-83829 discloses a conventional device for inspecting the height of bonded wires.
In the method used in this device, the correlation between wire height and wire width is investigated beforehand by changing the focusing level of an optical system, and the inspection level that constitutes the focusing level of the optical system is set from the upper-limit value and lower-limit value which act as standards for the height of the wire to be inspected. The acceptability of the wire height is then ascertained by determining whether or not the wire width obtained by imaging the inspected wire is within the range of the upper-limit value and lower-limit value in the correlation between wire height and wire width.
In this prior art, each wire is inspected; and therefore, if a defective wiring occurs, it is possible to ascertain the defective wire; and in addition, it is also possible to ascertain whether or not each wire is within a specific allowable range. In case the number of wires is small, it is possible to inspect the wire bonding process and wire inspection process on-line, thus making it possible to check all the wires.
On the other hand, since in the above prior art the acceptability of wires is ascertained by processing the images of individual wires, considerable inspection time is required if the number of wires is large; as a result, a sampling check is only available. However, when defective wiring occurs, with such a sampling check, there is a risk of a large number of defective products being generated at a later time as a result of the time lag between the wire bonding process and the inspection process.
In general, semiconductor assembly devices include, as shown in FIG. 6(b), island-down type devices in which an island 1a (which is an attachment part for a semiconductor chip 2) of the lead frame 1 is formed as a recess. In such island-down type devices, a height difference between the leads 1b of the lead frames 1 and the pads of the semiconductor chips 2 is extremely small. In FIG. 6(b), the reference numeral 3 indicates a wire connected between the pad of the semiconductor chip 2 and a lead 1b of the lead frame 1, and 4 indicates a package which are molded so as to protect the wire 3.
Thus, in the case of island-down type lead frames 1, there is almost no risk of the wires 3 shorting out on the edges of the semiconductor chips 2. Accordingly, a low wire loop shape (i. e., a loop shape in which the maximum height of the wire 3 is low) does not present any particular problems.
In the case of a high loop shape (i. e., a loop shape in which the maximum height of the wire 3 is high), however, there is a risk that the wire 3 will protrude from the package 4; accordingly, an inspection is necessary for such high loop shape wires. In cases where such island-down type lead frame 1 is inspected for high loop shapes, if the number of wires 3 is large and an inspection of all of the wires needs to be done, then the prior art described above cannot handle the inspection. Furthermore, since an expensive apparatus is required, there has been a demand for a device which can inspect all of the wires using an inexpensive apparatus.